[PATCH V12 3/7] dma: add Qualcomm Technologies HIDMA management driver

Sinan Kaya okaya at codeaurora.org
Fri Jan 15 09:16:55 PST 2016

>>> This doesn't seem to tie into KVM or VFIO, and as far as I can tell
>>> there's no mechanism for associating channels with a particular virtual
>>> address space (i.e. no configuration of an external or internal IOMMU),
>>> nor pinning of guest pages to allow for DMA to occur safely.
>> I'm using VFIO platform driver for this purpose. VFIO platform driver is 
>> capable of assigning any platform device to a guest machine with this driver. 
> Typically VFIO-platform also comes with a corresponding reset driver.
> You don't need one?

The HIDMA channel driver resets the channel before using it. That's why, I never
bothered with writing a reset driver on the hypervisor.

>> You just unbind the HIDMA channel driver from the hypervisor and bind to vfio
>> driver using the very same approach you'd use with PCIe. 
>> Of course, this all assumes the presence of an IOMMU driver on the system. VFIO
>> driver uses the IOMMU driver to create the mappings. 
> No IOMMU was described in the DT binding. It sounds like you'd need an
> optional (not present in the guest) iommus property per-channel

You are right. I missed that part. I'll update the device-tree binding documentation.

>> The mechanism used here is not different from VFIO PCI from user perspective.
>>> Given that, I'm at a loss as to how this would be used in a hypervisor
>>> context. What am I missing?
>>> Are there additional patches, or do you have some userspace that works
>>> with this in some limited configuration?
>> No, these are the only patches. We have one patch for the QEMU but from kernel
>> perspective this is it. 
> Do you have a link to that? Seeing it would help to ease my concerns.

The QEMU driver has not been posted yet. As far as I know, it just discovers the memory
resources on the platform object and creates mappings for the guest machine only. 

Shanker Donthineni and Vikram Sethi will post the QEMU patch later.

> Thanks,
> Mark.

Sinan Kaya
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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